In recent years, the aviation industry has been involved in a seemingly never ending quest to enhance the performance of modern avionics. One particular area in which avionics engineers have expended much effort, and achieved many successes, is in the substitution of completely electro-optical systems for the conventional mechanical systems of the past. A prime example of these efforts is in the area of inertial navigation systems.
Mechanical gyroscopes have been, for years, a central component to any inertial navigation system. Recently, the Helium-Neon (He-Ne) ring laser gyroscope has replaced many of the mechanical gyros. However, these He-Ne ring laser gyros have several disadvantages, including size, cost and weight. With the recent advent of the Global Positioning System, which utilizes satellite communications to assist inertial navigation systems, the fiber optic ring laser gyro is becoming an attractive component in modern inertial navigation systems.
A common problem appearing in any active ring laser gyroscope is phase locking between the counterpropagating beams. This phase locking often makes it difficult or impossible to detect low rotation rates.
One attempt to reduce the phase lock problem is discussed in U.S. Pat. No. 4,258,336 issued to Paul C. Fletcher, et al. and is entitled "Pulsed Ring Laser Gyro" and was issued on Mar. 24, 1981. This patent is hereby incorporated herein by this reference.
A major problem with the Fletcher approach is that it appears that several optical pulses will be oscillating in different modes in the optical cavity. This is a source of random variation or noise which has serious adverse effects on the operation of the gyroscope.
Consequently, there exists a need for an improved active ring laser gyroscope with reduced noise and concomitantly enhanced ability to detect low rotation rates.